1. Field of the Invention
This invention relates to manufacture of microcapsules. It more particularly relates to methods of manufacture of microcapsules for diverse uses such as carbonless paper and fragrance delivery substrate products. The manufacturing methods taught herein enable the production of novel resulting microencapsulated products. Preferred embodiments of the invention are in the fields of microencapsulation, and the process is especially useful in the fields of carbonless paper, pressure sensitive adhesives, pressure sensitive indicators, and fragrance delivery substrates.
2. Description of Related Art
Many processes for microencapsulation are known. These include methods for capsule formation such as described in U.S. Pat. Nos. 2,730,456, 2,800,457; and 2,800,458. Other useful methods for microcapsule manufacture are: U.S. Pat. Nos. 4,001,140; 4,081,376 and 4,089,802 describing a reaction between urea and formaldehyde; U.S. Pat. No. 4,100,103 describing reaction between melamine and formaldehyde; and British Patent No. 2,062,570 describing a process for producing microcapsules having walls produced by polymerization of melamine and formaldehyde in the presence of a styrenesulfonic acid. Microencapsulation is also taught in U.S. Pat. Nos. 2,730,457 and 4,197,346. Processes for forming microcapsules from urea-formaldehyde resin and/or melamine formaldehyde resin are disclosed in U.S. Pat. Nos. 4,001,140, 4,081,376; 4,089,802; 4,100,103; 4,105,823; 4,444,699. Alkyl acrylate—acrylic acid copolymer capsules are taught in U.S. Pat. No. 4,552,811. Each patent described is incorporated herein by reference to the extent each provides guidance regarding microencapsulation processes and materials.
U.S. Pat. No. 3,516,941 teaches microencapsulation processes in which the material to be encapsulated, or core material, comprising chromogen dissolved in an organic, hydrophobic oil phase is dispersed in an aqueous phase. The aqueous phase has dissolved wall forming material forming aminoplast resin which upon polymerization form the wall of the microcapsule. A dispersion of fine oil droplets is prepared using high shear agitation. Addition of an acid catalyst initiates the polycondensation forming the aminoplast resin within the aqueous phase, resulting in the formation of an aminoplast polymer which is insoluble in both phases. As the polymerization advances, the aminoplast polymer separates from the aqueous phase and deposits on the surface of the dispersed droplets of the oil phase to form a capsule wall at the interface of the two phases, thus encapsulating the core material. This process produces the microcapsules. Polymerizations that involve amines and aldehydes are known as aminoplast encapsulations. Urea-formaldehyde (UF), urea-resorcinol-formaldehyde (URF), urea-melamine-formaldehyde (UMF), and melamine-formaldehyde (MF), capsule formations proceed in a like manner. In interfacial polymerization, the materials to form the capsule wall are in separate phases, one in an aqueous phase and the other in a fill phase. Polymerization occurs at the phase boundary. Thus, a polymeric capsule shell wall forms at the interface of the two phases thereby encapsulating the core material. Wall formation of polyester, polyamide, and polyurea capsules proceeds via interfacial polymerization.
Common microencapsulation processes can be viewed as a series of steps. First, the core material which is to be encapsulated is emulsified or dispersed in a suitable dispersion medium. This medium is preferably aqueous but involves the formation of a polymer rich phase. Frequently, this medium is a solution of the intended capsule wall material. The solvent characteristics of the medium are changed such as to cause phase separation of the wall material. The wall material is thereby contained in the liquid phase which is also dispersed in the same medium as the intended capsule core material. The liquid wall material phase deposits itself as a continuous coating about the dispersed droplets of the internal phase or capsule core material. The wall material is then solidified. This process is commonly known as coacervation.
Gelatin or gelatin-containing microcapsule wall material is well known. Phase separation processes, or coacervation processes are described in U.S. Pat. Nos. 2,800,457 and 2,800,458. Encapsulations based on polymerization of urea and formaldehyde, monomeric or low molecular weight polymers of dimethylol urea or methylated dimethylol urea, melamine and formaldehyde, methylated melamine formaldehyde, monomeric or low molecular weight polymers of methylol melamine or methylated methylol melamine, as taught in U.S. Pat. No. 4,552,811 are incorporated herein by reference and preferable. These materials are typically dispersed in an aqueous vehicle and the reaction is conducted in the presence of acrylic acid-alkyl acrylate copolymers.
Microencapsulation processes typically rely on an emulsification and dispersion step. To be efficiently practiced on a large scale, large kettles and tanks become necessary.
A drawback of present methods for microcapsule production is that they tend to result in production of capsules of fairly wide size distribution. The smallest capsules, in many applications such as with carbonless copy paper production fall through the paper fiber interstices and are not available for imaging. As capsule size diminishes the wall material volume increases relative the volume of the core material resulting in capsules that often can be difficult to break.
Monosize capsules enable use of less wall material to coat the same amount of core material or internal phase.
The desirability of forming uniform capsules has been long recognized Rourke U.S. Pat. No. 5,643,506 teaches producing uniform capsules by minimizing residence time in the high shear zone of a rapidly rotating mixer blade.
Porous membrane materials although known and adapted for use in filters, colloids and as catalytic surfaces, have not been used in carbonless microcapsule production.
The present invention provides for an elegant means of capsule formation that enables more efficient formation of capsules and formation of a homogeneous microcapsule population having substantially uniform size distribution.